The present invention relates to a device for measuring a phase shift, which is not reciprocal, produced in an optical, ring-shaped interferometer.
Such an interferometer contains mainly a luminous energy source, generally formed by a laser, an optical device forming a wave-guide consisting either of a certain number of mirrors or of an optical fibre wound around itself, a device for separating and mixing the light and a device for detecting and processing the signal detected. Two waves coming from the separator device pass through the optical device forming the wave-guide in opposite directions.
A fundamental property of ring-shaped interferometers is their reciprocity, any disturbance with the optical path affects both waves in the same way.
However, there are two types of disturbances which affect this reciprocity. On the one hand, there are disturbances which vary within a length of time comparable to the propagation time of the waves along the interferometer optical path and, on the other, there are disturbances, called "non reciprocal", which have a different effect on the waves depending on the direction in which they move along the optical path.
Certain physical effects destroy the symmetry of the medium in which the waves propagate. In particular, there are the Faraday effect, or colinear magneto-optical effect, by which a magnetic field produces a preferential orientation in the spin of electrons in an optical material, this effect being used in the making of current measuring devices, and the Sagnac effect, or relative inertia effect, in which the rotation of the interferometer with respect to a Galilean ratio destroys the propagation time symmetry. This effect is used to produce gyrometers.
In the absence of disturbances which are not reciprocal, the phase difference .DELTA..PHI. between the two waves, which recombine in the separator and mixer device after passing through the optical path, is zero. The detection and processing device picks up signals representing the optical power of the compound wave obtained after recombination. If it is required to measure small amplitude disturbances, small rotation speeds in the case of gyrometers for example, the component due to the appearance of reciprocal disturbances varies little because the phase shift .DELTA..PHI. is almost zero. It is then necessary to introduce, artificially, an additional fixed phase shift or "non reciprocal bias" to increase the measurement sensitivity. However, this procedure meets with difficulties in its application, especially as far as stability is concerned the instability of the devices of prior practice is, in general, of the same order of size as the variations in the quantity to be measured. Methods designed to obtain greater stability in these devices have been suggested but the improvement in measurement sensitivity is less than that hoped for; the maximum theoretical sensitivity is determined by calculations of the limit due to quantum noise.